1. Field of the Invention
The present invention relates to a scanning beam antenna arrangement and, more particularly, to a scanning beam antenna arrangement capable of providing one or more spot beams which scan separate linear strips of the entire field of view of the antenna system, the arrangement comprising an optical system and a separate feed arrangement for each beam. Each feed arrangement comprises a linear phased array disposed within a parallel waveguide section with each section comprising an offset curved reflecting surface for bidirectionally reflecting a planar wavefront, at the array side thereof and scannable along one angular coordinate, into a converging wavefront at the other side thereof which is focused on the focal plane of the optical system and the principle ray thereof moves along a line at the aperture of the waveguide section and is always focused to a particular point on the image of the aperture of the optical system as seen by the feed arrangement.
2. Description of the Prior Art
Recently suggested designs for future generation satellite communication systems have proposed the use of one or more scanning spot beams at a satellite switching repeater for separately receiving and transmitting signals associated with a plurality of remote, spaced-apart, ground stations. One of the most recent designs, also forming the subject matter of a copending patent application Ser. No. 33,735, filed for A. Acampora et al on the same day as the present application and assigned to the same assignee, incorporates a satellite switching repeater which uses a plurality of linear scanning spot beams to concurrently scan along separate parallel strips of the overall ground service region of the satellite communication system in accordance with a predetermined communication sequence. Such system, however, would require a scanning beam antenna arrangement which can provide one or more linear scanning beams which are efficiently directed over separate strip regions of the overall service region.
Known scanning antennas have taken many forms. One such antenna is disclosed in U.S. Pat. No. 2,656,464 issued to C. V. Robinson on Oct. 20, 1953 which relates to a microwave scanning antenna of the "pill box" type arrangement. More particularly, the scanning antenna comprises a semi-parabolic reflector surface positioned at one end of a parallel-plate waveguide member; a movable waveguide feedhorn for illuminating the semi-parabolic reflector surface in the waveguide member; and means for mounting and pointing the feedhorn from the focus toward a point in the mid-region of the reflector and for moving the feedhorn along a portion of an arc of the circle which is tangent to the reflector at the point and passes through the focus, whereby a scanning output radiation beam is produced at the aperture of the waveguide member.
Another antenna arrangement is disclosed in U.S. Pat. No. 3,500,427 issued to S. Landesman et al on Mar. 10, 1970 which relates to a steerable antenna system for sweeping a given area of space with a first radiation lobe having high gain. The antenna system comprises a small panel of radiating elements associated with an optical system which may comprise two reflectors using either parts of cylindrical surfaces when scanning in only one angular coordinate is desired or parts of a quadric surface when scanning in both elevation and azimuth are desired.
Still another antenna arrangement is disclosed in U.S. Pat. No. 3,500,411 issued to J. D. Kiesling on Mar. 10, 1970 which relates to a retrodirective phased array antenna usable in a communication satellite and capable of providing several beams each of which may be electronically steered by an external beacon or ground station to provide scanning capabilities. The antenna comprises a reflecting surface and a plurality of antenna elements positioned near the focal plane of the reflecting surface. In operation, a beam from a ground station is reflected by the reflecting surface onto a portion of the antenna elements. Other antenna elements not illuminated by the beam are operated to emit a beam towards the same or other ground stations dependent on the properties of the received signal, e.g., the amplitude and phase.
Yet another antenna arrangement is disclosed in U.S. Pat. No. 3,766,558 issued to J. A. Kuechken on Oct. 16, 1973 which relates to a raster scan antenna capable of scanning in one or two planes. The antenna comprises a plurality of feed elements in a phased array where beam steering is effected by progressive phase shifts introduced to the various array elements. To effect the raster scan capability, a prime oscillator frequency f.sub.o is mixed with a variable steering oscillator frequency f.sub.s to produce sum and difference frequencies f.sub.o +f.sub.s and f.sub.o -f.sub.s. The latter is fed to a delay line having a plurality of taps spaced apart in accordance with the particular arrangement of array elements, the delay line providing in the frequency signal f.sub.o -f.sub.s progressive phase shifts from tap to tap. The individual tap outputs are separately mixed with the sum frequency signal f.sub.o +f.sub.s, wherein N separate outputs are simultaneously derived having the same frequency of 2f.sub.o but varying from one another progressively in phase. Variation of f.sub.s, which causes the phase relationship between the N outputs to change as they are fed to the radiators in one-to-one correspondence, thus provides a sweep of the beam in one plane. Raster type scanning is provided by introducing a multi-tapped secondary delay line at each tap of the primary delay line and mixing instead the tap outputs of each secondary delay line individually with f.sub.o +f.sub.s and applying the resultant 2f.sub.o outputs to individual ones of a corresponding element row of a two-dimensional array. The tapped spaacings on each of the secondary delay lines relative to the tapped spacings of the primary delay line is such as to provide scanning of an order of magnitude more sensitive in one plane.
In the article "Imaging Reflector Arrangements to Form a Scanning Beam using a Small Array" by C. Dragone et al in The Bell System Technical Journal, Vol. 58, No. 2, February 1979 at pp. 501-515, a small phased array is combined with a large main reflector and an imaging arrangement of smaller reflectors to form a large image of a small array over the main reflector. An electronically scannable antenna with a large aperture is thus obtained, using a small array disposed on a plane which is the conjugate plane of the main reflector plane. With such an arrangement, small imperfections in the reflecting surface of the main reflector can be corrected efficiently at the array.
The prior art scanning beam antenna systems, however, provide a scanning spot beam with either a two dimensional phased array or a linear phased array which radiates electromagnetic energy that is reflected from the entire reflecting surface of a main reflector. Where a linear phased array is caused to transmit a beam of electromagnetic energy which only reflects from a strip portion of the main reflector, only a fan beam is obtained which illuminates all of a strip portion of the entire field of view of the antenna system which fan beam can then be scanned across the entire field of view. The problem remaining in the prior art is to provide a feed system which produces a linearly moving spot beam along a strip portion of the entire field of view of the antenna system that is capable of being combined with other similar feed systems to use the same optical reflectors and provide a plurality of linear scanning spot beams which are scannable parallel to each other along different strip portions which strip portions cover the entire field of view of the antenna system.